Trunked digital radio communication systems are commonly used by public service organizations, such as police, fire, and ambulance squads, and by many private organizations to communicate with each other. Trunked digital radio systems provide an efficient means to communicate between single users and/or groups of users. They allow for one call to be made to many users simultaneously, such as a police dispatcher sending out a call to all officers at once. Any officer receiving the call has the ability to respond to the dispatcher, as well as to all other officers using the system. This makes these type of communication systems well suited for public safety and municipal applications.
Digital trunked radio systems comprise a plurality of radios that communicate with each other via a base station. An illustration of one example of a digital trunked radio system is shown in FIG. 1. FIG. 1 illustrates a system with one base station; however, a system can contain several base stations networked together over a wide geographic area, with each station known as a site. On large systems, each radio communicates with a site within its range of transmission, and switches between sites when the radio travels throughout the geographic area covered by the system.
Digital trunked radio systems operate by allowing a user to transfer a voice call (or data call) to another user or group of users on the system. The information is transferred on one of a plurality of channels, referred to as working channels. A control channel assigns a working channel to every transmission, and notifies both the transmitting radio and all receiving radios of the working channel assignment. When the transmission is completed, the assigned working channel is released, and thus becomes available for a new transmission.
Because each transmission within a single conversation could theoretically be sent on a different working channel, it was difficult for individuals outside of the system to “eavesdrop” on the users of the system using commercially available scanners. Preventing eavesdropping is an area of concern for many municipalities using digital trunked radio systems, as it is obviously critical that crucial communications between personnel (e.g., police officers) are not intercepted by individuals who are not authorized to receive them. Clearly, police officers do not wish for transmission regarding their location to be intercepted by the very individuals they might be pursuing.
As scanner technology has advanced, sophisticated scanners were developed that could keep up with the channel switching that occurs on digital trunked radio systems. These sophisticated scanners are able to understand the control channel communications and, as a result, can follow conversations as the transmissions switch from one working channel to another.
Existing systems have used various techniques to defeat the newer, sophisticated scanners, with varying levels of success. The Enhanced Digital Access Communications System (EDACS), produced and sold by M/A-COM Private Radio Systems, Inc. (Lynchburg, Va.), employs a tone-drop technique to make the calls more difficult to follow. This technique transmits a tone following a transmission on a working channel. The scanner remains locked on the tone, while the radio itself recognizes the tone and releases the working channel. However, while this technique worked for less sophisticated scanners, highly sophisticated scanners could also recognize the tone sequence and drop the working channel.
Another alternative is to use encryption devices to encode and decode each transmission (voice or data) such that the users of scanners would not be able to understand the communication, even if the scanner could follow the channel switching. This also has some drawbacks. First, encryption requires the use of fairly complex encryption algorithms to encode the transmissions, which then need to be decoded upon receipt. This requires additional hardware to be added at the base station and on each radio, or alternatively, requires the use of encoding and decoding encryption software. This is an expensive solution. In addition, each message must be encoded and decode, which further causes a strain on the system time constraints.
Another concern has developed over the years that digital trunked radio systems have been in operation. Radios that formerly were part of digital trunked radio systems and have been lost or stolen over the years have been appearing for sale at places such as Ebay (www.ebay.com). Radio hackers purchase these radios and program them to intercept transmissions on systems being used today. In addition, information regarding the system to which these “pirate” radios belonged can be read from the radios, such as the RF frequencies on which the system operates. This type of information makes it easier for unauthorized access to the communications, and thus adds another security concern in addition to the advancement of scanner technology.
Furthermore, as systems such as EDACS expand to include more sites and a wider geographic area, more personnel are required to maintain and administer the system. Unauthorized access to site data by individuals other than the proper administrators is another means by which system information can be obtained by unauthorized parties.
It is desired to prevent unauthorized individuals from accessing the radio system, either by intercepting transmissions or by obtaining the system information directly from a radio or from site equipment. What is desired is a simple, cost-effective security system that provides the required level of security in all facets of the system, including over the air transmissions and radio or system site access.